This invention relates generally to an electrophotographic printing machine, and more particularly concerns a color electrophotographic printing machine adapted to reproduce an original document containing two colors therein.
The process of electrophotographic printing comprises exposing a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive surface are discharged, recording thereon an electrostatic latent image corresponding to the original document. A development system, thereupon, moves the developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image. Thereafter, the toner powder image is transferred to a sheet of support material. After the toner powder image has been transferred to the sheet of support material, the sheet of support material advances to a fuser which permanently affixes the toner powder image thereto.
The foregoing briefly describes the basic concept of electrophotographic printing. Color electrophotographic printing utilizes this process to create successive, single color light images which, in turn, records single color electrostatic latent images on the photoconductive surface. These latent images are developed with toner particles complementary in color to the single color light image. The toner powder images are transferred to the sheet of support material in superimposed registration with one another forming a color copy corresponding to the original document. This is a subtractive system. Because of the inherent limitations found in most known colorants, it is generally necessary to employ costly and complex masking and/or balancing techniques to achieve a faithful color reproduction. Furthermore, because of the number of exposure and transfer operations involved, registration is frequently a problem in this type of system.
Many important applications of color do not require high fidelity. These are applications wherein color is employed functionally rather than esthetically, e.g. to distinguish, to contrast, or to emphasize as in diagrams, accounts or reports. In such applications, the only requirement is that the colors utilized may be easily and readily distinguishable from one another. Accurate matching and small differences are not significant. In these situations, a numerous choice of colors is not required. The color requirement is usually limited to two contrasting colors, i.e. blue and red, or black and red, etc. These colors will suffice in many applications, such as to distinguish profit and loss in accounting, to highlight amendments to a drawing or to a draft, and to emphasize points in a report. Such use is compatible with conventional typewriter machines utilizing two color ribbons and other convenient office practices.
Accordingly, it is a primary object of the present invention to improve electrophotographic printing by reproducing a copy of an original document in two colors when said colors are easily and readily distinguishable from one another.